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Thread: Weber Idle Screws

  1. #1
    Registered User Leif190's Avatar
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    Weber Idle Screws

    I have put on a pair of new Webers. A read a lot of info on how to sync them, but never got it right. The engine just wouldn't idle at sensible rpm's, so I bought a vacuum meter to investigate further (as pictured), which was a great help. I strongly recommend it for any carb enthusiast :-)
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    I saw that I could perfectly sync the two barrels in each carburetor, but it was impossible to harmonize both carbs. I removed the idle screws to check for any blockage, but I saw that the two carbs had very different idle screws (see picture).
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    When I bought the carbs the supplier told me they were set up for a 190 SL, but I wonder if he may have mixed up something here. Do any of you with Webers have different idle screws, and if so, should the longer ones be in the front or rear carb.

    I would be very grateful for any pictures of your idle screws.

    Here are pictures of all the four different progression holes. I believe they are as similar as they can be.
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    __________
    Leif Eriksrød
    1957 190 SL

  2. #2
    Registered User MBdrvr's Avatar
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    Re: Weber Idle Screws

    I had an issue with an idle screw that had broken off in the carb sealing the passage. The end of the idle screw was now flat sort of what you see on yours. New ones have a definite point.

    I believe that all the parts need to match from carb to carb.
    Paul Greenblatt
    1960 190SL
    1966 250SE Cabriolet
    2015 E400 Coupe

  3. #3
    Registered User Leif190's Avatar
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    Re: Weber Idle Screws

    Thanks, Paul. That makes sense and I will see if I can locate some broken off pieces down there. If that's the case, I've been fooled by the fact that the two screws look identical on each carbs. I'll post my findings :-)

    Where you able to get the broken piece out without removing the carb?
    __________
    Leif Eriksrød
    1957 190 SL

  4. #4
    Registered User MBdrvr's Avatar
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    Re: Weber Idle Screws

    I could not figure a way to release the tip of the screw from the orifice inside the carb. I bought a 1mm drill bit from Mcmaster Carr and drilled it out. I put in a new idle screw.

    My carbs were off the car. If you leave them on you may get some of the debris in the intake. Not good.
    Paul Greenblatt
    1960 190SL
    1966 250SE Cabriolet
    2015 E400 Coupe

  5. #5
    Registered User bsandefer's Avatar
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    Re: Weber Idle Screws

    I have a new set of webers on the car - and an old set - all of the idle screws are the same length. Let us know if you are on to something!
    Bill Sandefer
    1963 190SL
    New Orleans

  6. #6
    Registered User Leif190's Avatar
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    Re: Weber Idle Screws

    Paul, your diagnosis was spot on. The tips of the screws were broken off. And they were stuck so hard that there was no way I could remove them with the carb in place. I took the carb off and drilled them out, like you did. The metal in the carb body is however much softer than the tip, so I have accidentally expanded the holes a bit. I think I will try to shrink them again with solder tin while I wait for new screws to arrive.
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    Leif Eriksrød
    1957 190 SL

  7. #7
    Glen glengineer's Avatar
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    Re: Weber Idle Screws

    That first picture is four vacuum meters, all connected to the vac ports? And you adjust idle screws to balance them all out at the same time? That's a very good idea and not too expensive, I'll bet, the cost of 4 vac gauges versus the hours of adjusting and frustration that you save. I am stealing this idea!

  8. #8
    Registered User Leif190's Avatar
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    Re: Weber Idle Screws

    Yes, very convenient. It comes with a selection of tubes and fittings. I bought it here: https://www.aliexpress.com/item/Fuel...608.0.0.N09ZcK
    __________
    Leif Eriksrød
    1957 190 SL

  9. #9
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    Re: Weber Idle Screws

    It's common for the screws to damage the holes. My understanding is that the size and shape of these holes is pretty critical - that if damaged, the carb should be replaced. I could be incorrect, but that's what I recall.
    Bill Streep
    San Antonio, TX
    1957 190SL
    2009 CLK550
    2014 E550
    2014 E63s Wagon

  10. #10
    Registered User Leif190's Avatar
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    Re: Weber Idle Screws

    I will report back later if my repair is successful. I ended up filling the holes with chemical metal and then drill new ones with original diameter. Visually it looks OK.

    After everything was put together again I found that I have a major leak from the manifold gasket and plenty of air is sucked in there. So, I have to take it all off again and fix this before I get a final answer to my carb repair.
    __________
    Leif Eriksrød
    1957 190 SL

  11. #11
    Registered User Leif190's Avatar
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    Re: Weber Idle Screws

    Well, I have now replaced the manifold gasket and put everything back together. I have synchronized the linkages and the idle mixture several times. I have tried some different idle jets, main jets and pump jets. It runs fine on idle, but "coughs" around 2000 rpm under acceleration. It feels a bit "rough" after acceleration and stabilizing on on higher rpms, almost as it is running on three and a half cylinders. Since I have run out of new things to re-check on my new Webers, I wonder if I have another problem.

    So I took a long drive and then checked the spark plugs. My plug number four looks wet compared to the other. I would be grateful for opinions on these pictures, and ideas of what to check next. Is it likely that one wet plug is carb related when all jets are equal across the barrels?

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    __________
    Leif Eriksrød
    1957 190 SL

  12. #12
    Registered User Leif190's Avatar
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    Re: Weber Idle Screws

    Here's an update to round off this thread. I think my car runs pretty good now . After further fiddling there are two important things I will add here.

    I tried it out after reading all threads I could find about Webers here so the information is allready known to those of you who have been here for long.

    The first is the larger idle jets. I went up to 65F8, or "F8 65" which is outside the recommended range, but others have had success with these.

    The other is the ignition timing. I had to change this substantially compared to the standard 10 degrees at 1000 rpm.
    __________
    Leif Eriksrød
    1957 190 SL

  13. #13
    Administrator JimVillers's Avatar
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    Re: Weber Idle Screws

    Lief .... With the larger idle jet, you will probably have a flat spot in the 2500-3000 RPM range. That is when the fuel transitions from the idle jet to the main jet. You should check your timing at 3000 RPM or above (max centripetal advance); 38-40 degrees advance.
    Jim Villers
    1961 190SL, 230SL 5-speed, MGB 5-speed, Porsche 356C; 1967 Porsche 911; 1950 Jeep CJ3A

  14. #14
    Registered User Leif190's Avatar
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    Re: Weber Idle Screws

    Jim, I have no flat spots now. I can't see exactly what my timing is, but I have painted marks on 10 and 40 degrees on the pulley for the old setup. I loosened the distributor and turned it maybe 5 (?) degrees further and, voila, it ran smoothly on acceleration and I have good low range torque.
    __________
    Leif Eriksrød
    1957 190 SL

  15. #15
    Registered User Leif190's Avatar
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    Re: Weber Idle Screws

    I am back on this thread to correct myself, I don't want anyone to copy this. After running on this setup for a while the engine started to run bad. When I checked the plugs they were black and dry from carbon deposits. I cleaned the plugs and it ran fine again, but the plugs were dry black again after a short drive.

    The combustion was very inefficient from both to rich mixture and too retarded timing. I started with adjusting the timing back to Jim's recommended 38-40 degrees and using 50F9 idle jets. My flat spot between 2000 and 3000 rpms came back as expected. When I checked the plugs I saw that the carbon deposits were uneven. The cylinder 3 plug were still very black from carbon deposit, number 4 a little and 1 and 2 very grey as they should be.

    After further investigation I found out that the repair described earlier in this thread probably has made the hole under the idle mixture screw on the third cylinder a little larger than the others. I therefore tightened this screw much more than the other when I synchronized them again. I also opened up the bypass screw on cylinder 4 (which is generally recommended to be closed).

    After a test drive I had less flat spots, but still too much carbon deposit on number 3. I tried with leaner idle jets, 50F8, which gave clean plugs, but still flat spots. I then went up one size, to 55F8, which now seems to work well. So far I have clean plugs (only a short drive...) and it doesn't smell like an old gas station either. I have 135 main jets and 60 acceleration pump jets.
    __________
    Leif Eriksrød
    1957 190 SL

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